https://www.theguardian.com/technology/2018/may/31/twitter-blocking-users-who-were-underage-when-they-signed-up

Tech firm acts on accounts created by users under 13 at the time to comply with GDPR

In an effort to comply with GDPR, Twitter is blocking users who were underage when they signed up for the service – even if they’re now well over 18.